Conventionally, in the optical communication field and optical measurement field, devices have been studied that convert optical frequency by an arbitrary shift amount with high precision and at high speed. For WDM cross-connects in the optical communication field in particular, there are demands for optical frequency conversion devices with which a desired optical frequency component can be stably obtained, with which a shift amount of optical frequency can be fine-tuned, and with which little loss of optical intensity is incurred. For methods of converting the frequency of an input lightwave, a generally known method is that the frequency of an input lightwave is shifted by using an optical single sideband modulator (for example, NPL 1). Hereinafter, an optical frequency shifter disclosed in NPL 1 will be described briefly.
Referring to FIG. 1A, an optical single sideband modulator 10 includes two resonant type intensity modulators 30-1 and 30-2 that are provided to the arms of a main Mach-Zehnder (MZ) waveguide, respectively, and an optical phase shifter 32 that is connected to the resonant type intensity modulator 30-2 in series, wherein the resonant type intensity modulators themselves are made up of the MZ waveguide. A modulation signal is input to each of an input terminal RFA of the resonant type intensity modulator 30-1 and an input terminal RFB of the resonant type intensity modulator 30-2, and bias voltage for control is input to each of input terminals BiasA and BiasB thereof. A control signal for phase adjustment is input to an input terminal Phase of the optical phase shifter 32.
Referring to FIG. 1B, an optical frequency shifter 38 includes the above-described optical single sideband modulator 10, a modulation signal oscillator 11, an optical phase shift amount adjustment section 13, and a phase control section 14. The modulation signal oscillator 11 generates a modulation signal, and the modulation signal is directly input to the terminal RFA of the optical single sideband modulator 10 while it is input to the terminal RFB thereof through the phase control section 14. The modulation signal is assumed to be a sine wave of a single frequency f. The phase control section 14 is controlled so as to give a phase difference of −π/2 between the modulation signals to be respectively input to the terminals RFA and RFB of the optical single sideband modulator 10. Moreover, the optical phase shift amount adjustment section 13 controls the optical phase shifter 32 so as to give a phase difference of π/2 between an output lightwave of the resonant type optical intensity modulator 30-1 and an output lightwave of the resonant type optical intensity modulator 30-2.
It is assumed that continuous-wave laser light of a carrier frequency f0 that has a frequency spectrum as shown in FIG. 2A is input to the optical frequency shifter 38 having such a configuration. At this time, an output lightwave of the resonant type optical intensity modulator 30-1 of the optical single sideband modulator 10 has a frequency spectrum including frequency components of f0+(2n−1)f (n is an integer) in which the harmonics of the carrier wave and of even orders are suppressed as shown in FIG. 2B.
On the other hand, an output lightwave of the optical phase shifter 32, which has shifted an output of the resonant type optical intensity modulator 30-2 by π/2, similarly includes the frequency components of f0+(2n−1)f as shown in FIG. 2B, but of them, the phases of frequency components of f0+(4n−1)f are inverted by π. Hereinafter, if the phases of certain components are inverted in a regular manner even though a frequency spectrum is the same as another one as descried above, the formula of such frequency components will be represented by being enclosed by [ ]. Here, the frequency components of the output lightwave of the optical phase shifter 32 are represented by [f0+(2n−1)f] because only the phases of the frequency components of f0+(4n−1)f are inverted by π with respect to the frequency components of f0+(2n−1)f of the output lightwave of the resonant type optical intensity modulator 30-1.
When such output lightwaves of the resonant type optical intensity modulator 30-1 and of the optical phase shifter 32 are multiplexed, the frequency components whose phases are mutually inverted by π are canceled out, and a frequency spectrum including frequency components of f0+(4n+1)f as shown in FIG. 2C is obtained as an output lightwave of the optical frequency shifter 38. Accordingly, by using an optical band-pass filter having a characteristic as indicated by a dashed line in FIG. 2D, an output lightwave can be obtained in which the carrier wave frequency f0 is shifted to a frequency component of (f0+f) as shown in FIG. 2D.
According to the optical frequency shifter 38 as described above, the −3rd frequency component has high optical intensity as can be seen from FIG. 2C, and an optical band-pass filter is required to eliminate the −3rd frequency component. However, PTL 1 proposes an optical frequency conversion device that does not require such an optical band-pass filter.
A low-noise optical frequency conversion device disclosed in PTL 1 is provided with a driving system which includes a phase-locked tripler that triples the frequency of a fundamental wave, an amplitude adjustor that adjusts the amplitudes of the fundamental wave and the triple wave, and a 90-degree hybrid, thereby suppressing a third-order harmonic component among harmonics as shown in FIG. 2C. More specifically, this driving system applies a multiplexed wave of the fundamental wave and its triple wave to the terminal RFA of the optical single sideband modulator 10 shown in FIG. 1A and applies a multiplexed wave having a phase difference of −π/2 from that multiplexed wave to the terminal RFB. Then, the amplitudes and phases of these two multiplexed waves are appropriately selected, whereby the third harmonic is suppressed as shown in FIG. 2E.